Field of the Invention
The present invention relates to a new fiber-bundle-reinforced composite material having a ceramic matrix, a method for manufacturing a composite material and a method for manufacturing elements formed of a composite material.
Composite materials reinforced with high-temperature-resistant fibers and/or fiber bundles, having a ceramic matrix, have been known for about 10 years and are employed in many applications where extremely great requirements are set for a material. Such requirements may be a great temperature resistance and simultaneous strength and ductility.
The extent to which the fiber-reinforced and/or fiber-bundle-reinforced composite materials with a ceramic matrix, that are referred to below as CMC composite materials for short, can be used in the field of applications for which temperature resistance is required at high temperatures, depends not in the least on the structure of the matrix of the composite materials. As long as the matrix of the composite materials is composed of various phases, the matrix structure at the surface of the CMC composite materials can be damaged by eluting of a matrix phase which melts at lower temperatures and which can be attacked by chemical processes such as oxidation, as a result of which the period of use of the CMC composite materials is restricted nowadays. Those problems become all the greater if a CMC material is additionally exposed to mechanical abrasion. Upon that occurrence, new crystallites of the matrix which are set free all the time can be attacked at lower temperatures and are decomposed very quickly. Moreover, gaps in the matrix structure which have been originated by the eluted crystallites offer a possibility for increased mechanical attack. Furthermore, the structure of the matrix with respect to cracks also plays a part in stress acting on CMC materials by mechanical loading, because in the case of a matrix with cracks, matrix components can also be mechanically pulled out of the composite material much more easily.
Fields of application of CMC composite materials where the mechanical load plays a substantial part are, for example, the use of CMC elements as sliding bearing components and friction linings, such as brake discs and brake linings. First of all, in the field of friction linings, above all carbon-fiber-reinforced composite materials with a carbon matrix, so-called CFC composite materials, have been used. However, they had the disadvantage of a temperature resistance of the material, which is insufficient against an oxidative attack. Therefore, efforts are meanwhile being made to replace the carbon matrix of the composite material by a matrix which is more resistant to oxidation. In that respect, above all, the SiC matrix, which is resistant to oxidation at substantially higher temperatures (1500° C.), is used with and without an additional surface protection layer. Therefore, today the use, above all, of carbon-fiber-reinforced composite materials with a SiC matrix, referred to below as C/SiC composite materials, is provided for friction linings such as brake discs and brake linings.
In the meantime there are a number of methods for a manufacturing C/SiC composite materials, in particular even with the intention of using them as components of brake systems. Therefore, methods are described in German Published, Non-Prosecuted Patent Application DE 197 10 105 A1 and German Patent DE 197 11 829 C1 for manufacturing C/SiC composite materials where fiber bundles provided with at least one carbon layer or with a solidified layer of a binder are mixed with carbonaceous binders with or without additional fillers. The mixtures are then pressed and cured before they are carbonized, possibly graphitized and finally infiltrated with liquid silicon.
A modified method for manufacturing C/SiC composite materials is described in German Patent DE 197 49 462 C1. In that case a preform made of a fabric of carbon fibers is infiltrated in the first instance with a resin and subsequently cured. As already described regarding the other methods, a green body produced in that way is then carbonized, and infiltrated with liquid silicon.
All C/SiC composite materials manufactured heretofore according to the above-described methods have an inhomogeneous matrix structure which, in the case of the methods according to German Published, Non-Prosecuted Patent Application DE 197 10 105 A1 and German Patent DE 197 11 829 C1, manifests itself in the matrix having large cracks that follow a path between the individual fibers and/or fiber bundles of the composite material. That is caused by the clearly different coefficients of thermal expansion of the carbon fibers and the formed SiC of the matrix. As a result thereof, during the cooling of the siliconized samples, stress is induced in the matrix which relaxes due to the cracks (according to an article in Werkstoffwoche [Material Week] '98, Volume VII, p. 551). Moreover, the matrix also has phases of pure carbon and/or silicon and therefore has no homogeneous matrix composition. The carbon zones oxidize at higher temperature load, i.e. are burned-out out of the composite material, and the silicon zones are already melting at a temperature of about 1400° C. Therefore, those methods have heretofore failed to achieve a matrix structure of the C/SiC composite materials which is suitable for withstanding permanent temperature loads at high temperatures. In particularly, they have failed at additional mechanical load.
In addition, C/SiC composite materials manufactured according to the method of German Patent DE 197 49 462 C1 do not have a homogeneous matrix. In that method of manufacture large cracks are already deliberately generated in the matrix during the carbonization process. Those cracks are then filled during the final siliconizing with liquid silicon, which reacts with the carbon matrix to form SiC. However, carbon zones in the matrix which are not totally converted into SiC and the structure of the cracks in the matrix still exist.
With those conventionally used methods for manufacturing C/SiC composite materials, it has therefore heretofore not been feasible to manufacture a C/SiC composite material which has a fraction of the silicon and carbon phase as small as possible and as far as possible has no crack structure or a crack structure which does not have a negative effect during stress due to mechanical loading. A C/SiC composite material with a fraction of the silicon and carbon phase that is as small as possible can nevertheless be achieved with another method. In that method according to German Published, Non-Prosecuted Patent Application DE 197 36 560 A1 silicon carbide powder, which possesses a fine grain fraction with an average grain size of at most 2 μm and a coarse grain fraction with an average grain size between 1.5 μm and 30 μm, is mixed with the reinforcing fibers, then molded and subsequently sintered. Since a C/SiC composite material body with open pores is produced in that way, it has subsequently to be infiltrated with a carbonizable matter, then to be carbonized and finally, as usual, to be infiltrated with liquid silicon and to be siliconized. That method of manufacture indeed leads to the aim of an improved matrix, but compared with the other methods it has the disadvantage of requiring, even after the first formation of a SiC matrix, a further infiltration step with a carbon-donating material and a subsequent siliconizing, as a result of which an economical production of C/SiC composite materials is no longer provided through the use of the method. Moreover, the problem of the crack structure is not solved by that method.